Liquid electrical insulating compositions



Unite States Patent LIQUID ELECTRICAL INSULATING COMPOSITIONS Vernon Warner David, Chester, and Robert Irving, Great St. Helens, London, England, assignors to Shell Development Company, New York, N. Y., a corporation of Delaware No Drawing. Application June 18, 1956 Serial No. 591,781

Claims priority, application Great Britain AugustlZ, 1%5

4 Claims. (Cl. 252-63) This invention relates to an improved insulating oil composition and more particularly to insulating oil compositions which are stable, non-sludging and resistant to gassing.

It is well known that when hydrocarbon insulating oils used in electrotechnical apparatus, such as cables, transformers and capacitors, are subjected to high electrical stress at an oil/ gas interface, they tend todecompose producing, inter alia, gaseous products consisting mainly of hydrogen. This evolution of gas is undesirable since it may cause serious damage. Thus, bubbles of gas in the insulating oil permit local discharges in which electrical energy is dissipated as heat and in which the ions produced in the discharge and accelerated by the local electric field bombard the oil or its vapor withconsequent further decomposition of the oil and eventual failure of the apparatus.

Highly refined oils which consist mainly of saturated hydrocarbons tend to evolve relatively large volumes of hydrogen containing gas when subjected to electrical stress at an oil/ gas interface. This gas evolution can be inhibited by the presence in the oil of aromatic hydrocarbons whose vapor pressures permit them to be present in sufiicient concentration in the vapor phase. The amount of aromatic hydrocarbons which can be present for this purpose is, however, limited by the adverse effect of the aromatic hydrocarbons on the aging and sludging tendencies of the oil. In consequence electrical oils containing aromatic hydrocarbons produced by refining techniques generally do not contain sufiicient aromatic hydrocarbons of suitable vapor pressure to inhibit gas evolution completely although the total content of aromatic hydrocarbons may be relatively high, for example, up to 20%, depending on the nature of the original oil. The use of unsaturated aliphatic hydrocarbons to inhibit gas evolution, which has also been proposed, has similar disadvantages and insulating oils containing unsaturated aliphatic hydrocarbons are not in general produced in refinery practice.

It has now been found that an insulating oil which is gas evolving under conditions of high electrical stress can be converted into a gas absorbing oil by addition to an insulating oil, a minor amount of a non-aromatic cyclopentene compounds having a boiling point above 150 C. at atmospheric pressure and a vapor pressure greater than that of the base oil such as dicyclopentadiene and C -C alkyl fulvenes.

The gas evolving electrical insulating oil used in the compositions of the invention is one of the usual types of insulating oils commonly employed in electrical cables, switches, transformers, capacitors and other electrotechnical apparatus. It may be a highly refined, substantially saturated oil or a petroleum distillate fraction which has been refined by solvent extraction and treatment with sulfuric acid and adsorbent earth and contains a proportion of aromatic hydrocarbons.

Efiective non-aromatic containing cyclopentenes for use in the compositions of the invention are dicyclopentadiene or alkyl-substituted cyclopentadienes, and more particularly cyclopentadienes obtained by replacing one or more hydrogen atoms of the cyclopentadiene ring by an alkyl group or groups, which substances are effective in producing an oil of the desired gas absorbing characteristics. Compounds of this type include dicyclopentadiene, methylisobutylfulvene, diisobutylfulvene, octylfulvene and the like.

Particularly suitable compounds of this type are the fulvenes which are obtainable by the alkali-catalyzed condensation of aldehydes and ketones with cyclopentadiene, as for example, dimethylfulvene, B. P. 46 C. at 11 millimeters of mercury pressure, methylethylfulvene, B. P. 185 C. and methylisobutylfulvene, B. P. 78 C. at millimeters of mercury pressure.

The cyclopentenes in general constitute from about 0.01% to not more than about 2% and preferably from less than 1% and usually from 0.2% to 0.5% by weight of the compositions of this invention.

The terms gas evolving and gas absorbing as used in this specification and the claims mean that the oil, when heated to 50 C. and subjected to an applied voltage of 10 kv. in the Thornton modified gassing apparatus has respectively a positive or a negative gassing coefficient as defined by the equation Gassing coefficient= where V and V are the volumes of gas (in milliliters) evolved at '60 minutes and 10 minutes, respectively. The Thornton modified gassing apparatus is a modification of the B. P. M.-Pirelli Apparatus. Particulars of both apparatus and of the experimental procedure are given in an article in the Journal of the Institute of Petroleum, 1949, 35, 735-753, particularly pages 736-7 and 740-1, by G. H. Beavan, I. A. Cockburn and C. N. Thompson and entitled The evaluation of gassing tendencies of insulating oils: Apparatus, procedures and eifect of experimental variables. It is to be noted, however, that the gassing coefficient is defined in this specification by the equation given above and not as given in this article.

The effectiveness of the above particular cyclopentenes in promoting gas absorbing characteristics in a normally gas evolving electrical insulating oil is illustrated in the following Table l which summarizes the results of tests carried out in the Thornton modified gassing apparatus referred to previously in this specification. The base oil used in these tests was a solvent refined, acidand adsorbent earth-treated petroleum distillate having a viscosity of approximately 45 seconds Redwood I at F. and containing about 11% of aromatic hydrocarbons.

Table I milliliters per minute Concentration, percent wt.

Gassing 1 (ml./minute) Additive None Metgylisobutylfulvene 1 Defined by the equation:

Vac-V10 Gasslng coeliicient= milliliters per minute cording to the procedure set out in British Standards has a higher acidity than the other oils. The addition of only 0.5% by weight of dicyclopentadiene to the oil of 11% aromatic content is, however, sufficient to convert the-gas evolving oil into a gasabsorbing oil which, more-' over, has satisfactory oxidation characteristics.

TABLE II B. S. 148: 1951 oxidation characteristics and gassing properties of certain insulating oils Gassing Oil Coeflicient Sludge Acidity (mL/min.) (percent (mg.

Wt.) KOH/g.)

Oil containing 11% aromatics.---- 0. 73 1. 25 +0. 011 011 containing 11% aromatics+ 0.5% wt. dicyclopentadienefln. 0. 82 1. 20 0. 022 Oil containing 30% aromatics. l. 67 1. 75 -0. 012

B. S. 148: 1951 Specificationlimita. 1.1 (ESL) 2.5 (max) Other compositions of this invention which possess nonsludging and non-gassing properties include. transformer oils containing from 0.2% to 0.5% by weight of dicyclopentadiene, C -C alkyl-substituted cyclopentadiene or C -C alkyl-substituted fulvene and mixtures thereof.

The compositions of the invention possess satisfactory oxidation and electrical characteristics. desired, how:

ever, minor proportions of from about 0.01% to about 1%, as for example, 0.3 percent by weight, of a suitable anti-oxidant may be incorporated, such as 2,6-di-tert.- butyl-4-methylphenol or 2,4-dimethyl-6-tert.-butylphenol.

Advantageously also, a minor amount of a corrosion inhibitor may be incorporated, for example, an oil-soluble compound capable of ichelating metal ions.

We claim as our invention: v

1. A liquid dielectric composition comprising a major amount of a gas evolving electric insulating solvent refined, acid treated and adsorbent earth treated mineral oil containing from about 0.01% to about 2% of a nonaromatic cyclopentene compound selected from the group consisting of- C C alkyl fulvene and dicyclopentadiene.

2. A liquid dielectric composition comprising a major amount of a gas evolving electric insulating solvent refined, acid treated and adsorbent earth treated mineral oil containing from about 0.01% to about 2% of a C -C alkyl fulvene.

3. A non-gassing liquid dielectric composition comprising a major amount of an-electric insulating solvent refined, acid treated and adsorbent earth treated mineral oil containing from about 0.01% to about 2% of dicyclopentadiene.

4. A. non-gassing liquid dielectric composition comprising a major amount of an electric insulating solvent refined, acid treated and adsorbent earth treated mineral oil containing from about 0.01% to about 2% of methylisobutylfulvene.

References Cited in the file of this patent UNITED STATES PATENTS 2,112,735 Clark Mar. 29, 1938 2,231,248 Bowden Feb. 11, 1941 2,734,031 McNaughton Feb. 7, 1956 

1. A LIQUID DIELECTRIC COMPOSITION COMPRISING A MAJOR AMOUNT OF A GAS EVOLVING ELECTRIC INSULATING SOLVENT REFINED, ACID TREATED AND ADSORBENT EARTH TREATED MINERAL OIL CONTAINING FROM ABOUT 0.01% TO ABOUT 2% OF A NONAROMATIC CYCLOPENTENE COMPOUND SELECTED FROM THE GROUP CONSISTING OF C1-C8 ALKYL FULEVENE AND DICYCLOPENTADIENE. 